Industrial type switches, such as transfer switches, contacters, circuit breakers, and interrupters, are required to handle heavy electrical loads, typically comprising hundreds and even thousands of amperes at voltages ranging up to 15,000 volts AC and 1,500 volts DC.
Every switching operation produces arcing between the cooperating switch contacts, and when the electrical load being handled is heavy, the arcing created has the potential to cause considerable damage to the contacts. To mitigate this problem, it is customary to protect the main contacts, which carry the load, with arcing contacts which open after the main contacts open and close before the main contacts close. Thus, in theory, the arc is drawn only between the arcing contacts and not between the main contacts. The latter are formed of relatively soft, good conducting metals, such as silver or copper, which have low melting points and hence are very susceptible to damage by uncontrolled arcing. The arcing contacts are made of materials, such as refractory metals, e.g., tungsten, capable of withstanding extremely high temperatures while experiencing only moderate erosion. Nevertheless, the sacrificial activity of arcing contacts does eventually lead to damage, but they are easily field replaceable.
A problem develops due to the fact that the impedance of the arcing contacts is much higher than that of the main contacts. As a result, during opening of a switch, when all current flowing through the main contacts is transferred to the arcing contacts, a voltage drop is created across the main contacts while the air gap between them is still quite small. Consequently, on occasion, an arc is engendered between the separating main contacts potentially damaging to those contacts. If the current level is sufficiently high, unacceptable damage can occur rendering the main contacts unusable, thereby requiring major repair or replacement of the switch. The problem becomes more acute if, at the critical moment of main contact separation, the arcing contacts bounce or are otherwise disturbed in a manner which increases their impedance.
Environments in which the damage of arcing between the main contacts is particularly severe are those in which the switch is called upon to handle excessive motor loads or transformer inrush currents which are by their nature highly inductive. Thus, when switching highly inductive loads on a DC circuit, such as traction motors in a typical transit system, it is important to ensure as much as possible that all arcing takes place across the arcing contacts and none across the main contacts.
Even when an arc is confined to the arcing contacts, it is important that the arc be extinguished as quickly as possible. Rapid quenching of the arc not only minimizes erosion of the arcing contacts, but reduces the opportunity for the arc to jump into the gap between the opening main contacts.
From the discussion above, it will be appreciated that arcing across, and consequent damage to, the main contacts can be eliminated or at least greatly reduced, by insuring as large an air gap as possible between the opening main contacts before the arcing contacts begin to separate, and by thereafter insuring very rapid opening of the arcing contacts.